1. Field of the Invention
The present invention relates to a lens-interchangeable digital camera system and an interchangeable lens.
2. Description of the Related Art
Known digital cameras use an optical low-pass filter (hereinafter referred to as an optical LPF) of crystal or the like, having double refraction, arranged in front of an image pickup element to reduce false color (such as moiré) of a high-frequency component of a shot image. A single-lens reflex digital camera having a lens (a lens barrel) interchangeable with respect to a camera body includes image pickup elements different in pixel number namely in pixel pitch so that a plurality of interchangeable lenses within the same specification are mounted.
In the single lens reflex digital camera body, the optical LPF having a thickness responsive to a pixel pitch is mounted in front of the image pickup device having a different pixel pitch.
The thickness of the optical LPF is thus determined in accordance with the pixel pitch of the image pickup element. It is because rays of light transmitted through the optical low-pass filter are split into a plurality of rays by the double refraction property of an optical member forming the optical low-pass filter and the amount of splitting depends on the thickness of the optical member. To reduce the generation of moiré, a spatial frequency component of an image approximately matching the pixel pitch of the image pickup element needs to be eliminated. To this end, the amount of splitting of the rays of light must be set to be substantially equal to the pixel pitch. The thickness of the optical low-pass filter thus must be varied in accordance with the pixel pitch.
If the optical LPF having a different thickness is mounted in front of the image pickup element, the forming position of an image is moved. FIG. 17 is an optical path diagram illustrating the image-forming position that changes in response to the presence or absence of the optical LPF arranged in front of the image pickup element in a photographic optical system. A plane plate is arranged between the interchangeable lens and the image pickup element. The plane plate is a protection glass or the like that hermetically seals a dust-preventive filter, an infrared absorbing glass, and the image pickup element.
When an optical LPF 102 is not provided in front of an image pickup surface 103 of an image pickup element as shown in FIG. 17, the center luminous flux having passed through a lens 101 forms an image at a point P1 on the image pickup surface 103. A peripheral luminous flux having passed through the lens 101 forms an image at a point P2 on the image pickup surface 103. However, when the optical LPF 102 is provided in front of the image pickup surface 103 of the image pickup element, the center luminous flux having passed through the lens 101 forms an image at a point P1′ behind the image pickup surface 103. The peripheral luminous flux having passed through the lens 101 form an image at a point P2′ behind the image pickup surface 103. When the optical filter 103 is thicker, the luminous flux forms an image in a more rear direction. In other words, the effective optical path length of a luminous flux to the image-forming position depends on the thickness of the optical LPF. In a digital camera system having a plurality of camera bodies, the effective optical path length varies if the optical elements, such as the dust-preventive filter, the infrared absorbing glass, and the protection glass, used in the camera body change in thickness, or if the optical refractive indices vary, or if some optical elements are not used at all.
The optical length generally differs between a luminous flux incident on the center of the photographic screen and a luminous flux incident on the edge portion of the photographic screen in a photographic optical system of a digital camera, generating a curvature-of-the-field aberration as a result. The difference in optical path length of the center and edge portion of the photographic screen can be corrected by providing an optical unit formed by particular optical elements in the photographic optical system with an optical characteristic for canceling an aberration such as curvature of the field to correct the curvature of the field, spherical aberration, astigmatism or the like, as a result. In this way, an image is formed on the image pickup element.
However, when the aberration correction is applied to the lens interchangeable type single lens reflex digital camera body, the problem below may arise. That is to say, in a digital camera system including a first camera body having a first optical unit and an interchangeable lens designed for the first camera body, the interchangeable lens may be mounted on a second camera body having a second optical unit. The camera bodies are different from each other in effective optical path length because the optical elements of the first and second optical units are different in refractive index and thickness, and because a particular optical element is not used. The aberrations, such as the curvature of the field, cannot be appropriately corrected. More specifically, when an image pickup device is mounted, the thickness of the optical LPF needs to be adjusted to match the pixel pitch.
In known single lens reflex digital cameras, a lithium niobate (LN, LiNbO3) element is used instead of the crystal plate and the LN is significantly thin and larger in double refraction than the crystal plate. The number of pixels is changed by modifying the size of the image pickup device in order to keep the pixel pitch as it is. Because of its large double refraction, even a thin LN element permits the interchangeable lens to be used among the digital camera bodies different in pixel pitch without changing the effective optical path length of the luminous flux.
FIG. 18 is a plot of a relationship between a pixel pitch of the image pickup device (the number of pixels in the image pickup device) and a thickness of the optical LPF (for the crystal and LN element). As shown in FIG. 18, the thickness of the usable optical LPF becomes thinner as the pixel pitch P becomes narrower. Given the same pixel pitch P, the thickness of the usable LN element is about ⅕ to ⅙ of the thickness of the crystal.
Techniques to control effective optical path length using the optical LPF and to control spatial frequency characteristic in known digital cameras are disclosed in Japanese Unexamined Patent Application Publications Nos. 7-123421, 2000-244821, and 2004-112661 and Japanese Patent No. 2552855.
An image pickup device disclosed in Japanese Unexamined Patent Application Publication No. 7-123421 includes a switch circuit that switches between a first image pickup mode for forming a natural color image signal using an electronic signal outputted from an image pickup element and a second image pickup mode for forming an image signal having a high resolution with respect to a single color-or monochrome image. In response to the switching action, the optical LPF is inserted or retreated, and a variation in the effective optical path length is corrected by another optical system. The image pickup device provides a high-resolution image signal of a monochrome image.
An image pickup device disclosed in Japanese Unexamined Patent Application Publication No. 2000-244821 switches between an optical path compensating dummy glass in an LPF block and an optical LPF in response to a drive mode of an image pickup element. The image pickup device efficiently controls the generation of a false signal generated by a change in spatial sampling characteristics caused by a special driving of an image pickup element.
In an image pickup device disclosed in Japanese Patent No. 2552855, compensating optical means having the same effective optical path length as the optical LPF is inserted into an optical path instead of the optical LPF when a frame read mode is selected. The image pickup device is free from aliasing distortion even if sampling vertical scanning frequency is changed from 525 to 262.5. Image quality is thus maintained.
In accordance with a technique disclosed in Japanese Unexamined Patent Application Publication No. 2004-112661, a plurality of optical plane-parallel members are arranged on light incident sides of a plurality of image pickup devices having respectively image pickup elements different in pixel pitch. The sum of thicknesses of the plurality of optical members along an optical axis is set to be equal among the plurality of image pickup elements. The spherical aberration and astigmatism of an interchangeable lens thus remains unchanged among image pickup devices.